Heat exchangers consisting of u-shaped steel pipes located within a cased borehole are presentlybeing used at several locations in the U.S. for extraction of geothermal energy from relativelyshallow geothermal resources. The applications are principally space and domestic water heatingwith a few commercial and small industrial uses (1). In Klamath Falls, Oregon, over 400 ofthese downhole heat exchangers (DHEs) are in use. Several similar designs are in use in Vale,Oregon, and over 100 of somewhat different design are installed in Reno, Nevada.

Fig. 1 shows a typical installation of a DHE in Klamath Falls and the type to be consideredhere. It consists of (i) the wellbore, generally 15 to 30 em (6 to 12 in.) in diameter, (ii) acasing sealed to the wellbore at the top end for a distance of about 6 m (20 ft) and perforatedat two levels; at the hot water strata level, located by the well drillers, (at the well bottom)and just below the standing water level, and (iii) the u-shaped heat exchanger made from baresteel pipe.

It has been known for many years that a well of this design, with the casing perforated asindicated, has a heating capacity several times that of a more conventional well with a solidcasing. Since the bore is somewhat larger than the casing and the perforations allow fluidflow between the inside of the casing and the outside of the casing, it was believed that a convectioncell was established. A recent study by the authors (2,3,4) has experimentally verifiedthe existence of such a circulation cell and modelled the flow and heat transfer within the wellbore.These will be discussed below in relation to the performance of DHEs.

The DHEs in Reno for the most part have solid casings and presumably much less capacity althoughlittle testing of these has been done. There appears to be a resistance to a perforatedcasing in the Reno area because of the possibility of sloughing of'the well wall around the casing.This sloughing does not appear to be a problem in the Klamath Falls area.

Fig. 2 illustrates the typical use of a DHE at a Klamath Falls residence. Standard hotwater heating systems are connected in a closed loop with the DHE and the loop filled with domesticwater. The terminal units may be wall convectors, baseboard units, radiant floor or ceilingpanels, or more usually fan coils in forced air systems. Some systems use only gravity circulationfor the closed loop while others use a circulating pump. A separate smaller DHE at citywater pressure is usually used for domestic hot water.